In the design and manufacture of satellite propellant and pressurant tank shells there are two main driving characteristics. These are weight and mounting configuration. The weight, or total mass of the finished item is controlled through the use of lightweight high-strength materials to minimize the thickness of the material while still meeting the operating pressure requirements. This generally drives the designs toward spherical tanks with thin membrane thicknesses or cylindrical tanks with hemispherical ends. These tanks are attached to the spacecraft structure such that the mechanical loads from the mass of the tank and its contents are transferred to the frame of the satellite. This requires that the attachment points are thicker than the nominal thickness of a tank that only must carry its low pressure loads.
In addition there is often the need to mount surface tension propellant management devices or other fluid expulsion devices such as bladders or bellows inside the tank. There also might be needs for local stiffening rings for stabilization of the pressure shell for vacuum conditions or high external buckling loads.
To solve this issue, designers typically provide thick section attachment points at selective locations such as at polar bosses, skirts at the circumference of the tank, or bosses attached to the sides of the tanks. These attachment features drive the initial material thickness of the raw materials from which the tanks are fabricated. To illustrate this consider a typical tank used for propellants of a GPS space satellite.
The raw material for the domes of such tanks are often forgings with sufficient thickness that an integral circumferential attachment ring can be machined from the parent material. The added thickness of the forging significantly increases the material cost. Additionally the increased material thickness significantly increases the fabrication cost as additional machining time and operations are required to remove the thicker material in the locations where it is not required. To reduce these inefficiencies, some designs utilize thinner domes welded to attachment rings fabricated from forgings, thereby reducing the total amount of material removal required, but increasing the fabrication costs and overall mass by producing and joining the welded ring to the tank components.
Another method of reducing the cost of manufacturing these tanks is the use of spinformed domes in lieu of forgings. The spinformed domes take advantage of the ability to spinform domes from thinner sheet metals, thereby reducing the amount of waste material at the “machine blank” stage, but the thinner sheet cannot generally accommodate the thicker attachment points except at the polar bosses. To generate the thickness required for attachment bosses, generally a thicker sheet is used and contours are machined in the part before spinforming, or a thinner sheet is locally reinforced through techniques such as inertia friction welding bar stock onto the sheet at the boss locations. But it is generally not possible to fabricate circumferential attachments on domes made from spinformed processes without adding welded attachment rings fabricated from forgings.
Therefore, there is a need for a method of fabricating space satellite tank components that is capable of producing physical and mechanical material characteristics in complex shapes with minimal excess material to be removed in subsequent machining operations.